The present invention relates to a digitally implemented method for determining basement depths of 3-dimensional density interfaces from gridded gravity data. The density contrast above the interface is assumed to be varying parabolically with depth.
It has been known since the time of Sir Isaac Newton that bodies having mass would exert a force on each other. Therefore, from measurements of gravity fields, it could be possible to identify large objects having a change in density even the objects are buried beneath the earth""s surface. When objectives are meant for regional geological and hydrocarbon explorations, lateral variation in density between the sedimentary and basement rock mass is generally responsible for the measured gravity fields. In one article, The use of rapid digital computing methods for direct gravity interpretation of sedimentary basins, Geophysical Journal of the Royal Astronomical Society, Vol. 3, 1960, Bott has suggested a simple but ingenious method to trace the floor of a sedimentary basin from the measured gravity fields. This method of interpretation involved the approximation of a sedimentary basin by a series of two-dimensional juxtaposed blocks having uniform density. However, in nature, many sedimentary basins on the continental platform have limited strike lengths, and therefore, approximation of such basins by geophysical geometries having limited strike lengths are often justified. Further, copious of evidence exists to show that the density of sedimentary rocks increases with depth.
In two articles, The development and use of a high precession down hole gravity meter, Geophysics, Vol. 31, No. 4, 1966 by Howell et al and, Variation of density with rock type, depth, and formation in the Western Canada basin from density logs, Geophysics, Vol. 45, No. 6, 1980 by Maxant, it was shown that the density of sedimentary rocks varies with depth. In two more articles, Three-dimensional interpretation of gravity data from sedimentary basins using an exponential density-depth function, Geophysical Prospecting, Vol. 35, No. 9, 1987 by Granser and, Gravity inversion of an interface above which the density contrast varies exponentially with depth, Geophysics, Vol. 53, No.6, 1988 by Chai et al, it was shown that the decrease in density contrast of sedimentary rocks with depth could be simulated by an exponential density-depth function. In a recent article, INVER2DBASE-A program to compute basement depths of density interfaces above which the density contrast varies with depth, Computers and Geosciences, Vol. 27, No. 10, 2001, Chakravarthi et al opined that at least it is not possible in the space domain to derive analytical gravity expressions of geophysical geometries with an exponential density-depth function. Further, it was shown that in spite of the existence of theoretical gravity solutions in the frequency domain with an exponential density-depth function, truncation errors would cause serious problems while transforming theoretical gravity fields from frequency to space domain. In the same article it was further shown that the parabolic density-depth function could be one of the alternatives to simulate the decrease in density contrast of sedimentary rocks with depth and such a simulation could make it possible to derive closed form analytical gravity expressions in the space domain. In the present invention, the parabolic density function is used to develop a method of interpretation and related computer code to optimize the gravity fields of three-dimensional subterranean density interfaces.
Bott, M.H.P., 1960, The use of rapid digital computing methods for direct gravity interpretation of sedimentary basins, Geophysical Journal of the Royal Astronomical Society, Vol. 3, P 63-67.
Howell, L. G., Heintz, K. O., Barry, A. 1966, The development and use of a high precession down hole gravity meter, Geophysics, Vol. 31, No. 4, P 764-772. Maxant,J. 1980, Variation of density with rock type, depth, and formation in the Western Canada basin from density logs, Geophysics, Vol. 45, No. 6, P 1061-1076.
Granser, H. 1987, Three-dimensional interpretation of gravity data from sedimentary basins using an exponential density-depth function, Geophysical prospecting, Vol. 35, No. 9, P 1030-1041.
Chai, Y. J., Hinze, W. J., 1988, Gravity inversion of an interface above which the density contrast varies exponentially with depth, Geophysics, Vol. 53, No. 6, P 837-845.
Chakravarthi, V, 1995, Gravity interpretation of non-outcropping sedimentary basins among which the density contrast decreases parabolically with depth, Pure and Applied Geophysics, Vol. 145, No. 2, P. 327-335.
Chakravarthi, V., Singh, S. B., Ashok babu, G., 2001, INVER2DBASExe2x80x94A program to compute basement depths of density interfaces above which the density contrast varies with depth, Computers and Geosciences, Vol. 27, No. 10, P. 1127-1133.
The primary object is to accurately simulate the density contrast depth dependence of sedimentary sequence in a sedimentary basin and henceforth to establish a comprehensive formulation for future activities.
Another object is to obtain accurate depth estimates to the floor of a sedimentary basin from the measured gravity fields.